In particular an electric motor (e.g. an asynchronous motor) can be regarded herein as the load. A thermal overload on the load arises, for example, through mechanical overloading of an electric motor or the failure of a conducting route (phase) of the electric motor. This leads to undesirable heat generation at the load, which ultimately can lead to damaging of the load.
In order to detect an imminent thermal overload on an electrical load, devices are typically integrated into the conducting route via which the load is supplied with electrical energy, so that an imminent thermal overload on the load can be detected with said devices. Single-phase or multi-phase monitoring can take place, that is, a single conducting route (one phase) or a plurality of conducting routes (multiple phases) of the load can be monitored.
The respective devices have a current path for each conducting route to be monitored, by which the energy supply taking place via the conducting route is guided. The electrical energy of the load is therefore fed via the current path through the device. By way of a monitoring arrangement of the device, the current flow in the current path is monitored so that an imminent overload on the load can be detected. Devices of this type are, for example, overload relays or circuit breakers. Apart from protection against thermal overload by way of an A-release, a circuit breaker for a downstream load also has a short-circuit protection by way of an N-release.
In the present application, overload protection, in particular, is to be provided for a load (e.g. motors, lines, transformers and generators).
A variety of requirements are placed on a device for determining a thermal overload on a load:                The device should, as far as possible, be able to monitor both AC and DC currents so that both AC and DC loads can be monitored for overload.        The device should have the largest possible setting range. The setting range is the range within which monitoring of the operating current of the electrical load can take place. Said setting range is limited by the operating current upper limit Io and the operating current lower limit IU (Io to Iu). Using a setting device (e.g. a setting screw) at the device, the thermal overload release can be set to the relevant nominal current of the load, so that targeted monitoring of the downstream load to be monitored can take place.        The device should generate the smallest possible power loss.        The device should have the simplest possible electrical isolation between the current path to be monitored and the monitoring arrangement which detects the overload.        The device should have a thermal memory. I.e. if an imminent thermal overload on a load is detected, the current feed to the load should be interrupted for long enough until cooling of the load is assured. Thus, no immediate connection of a load should be enabled following the determination of a thermal overload.        
If a thermal overload on a load is imminent, an increased current rise takes place in the individual conducting routes (phases) of the load. A device connected upstream of the load for monitoring a thermal overload on the load can therefore detect and evaluate this increased current rise by monitoring the current path thereof. Different measuring principles can be used for this. Detection of an imminent overload can therefore be carried out with different monitoring arrangements of the device. Monitoring arrangements for detecting an overload on a load particularly include a bimetallic release, a current transformer or a shunt on the corresponding current path for each phase of the load to be monitored.
In the case of monitoring by way of a bimetallic release, the current path to be monitored is coupled to a bimetallic release in such a way that, due to the current rise, heating of the bimetallic release, and ultimately a spatial deflection of a part of the bimetallic release, takes place. This deflection is detected and further evaluated. Using a bimetallic release, both direct currents and alternating currents can be detected. The typical setting range of the bimetallic release is from 1 to 1.6. A disadvantage of the bimetallic release is that it generates a high power loss. Thermal memory and electrical isolation between the individual conducting routes (phases) can be achieved with little effort in the case of bimetallic releases.
In the case of monitoring by way of a current transformer, the respective current transformer detects the current flow in the current path thereof, so that an evaluation unit can carry out a further analysis of the current flow and ultimately detect an imminent overload. A disadvantage of this measuring method is that DC currents cannot be detected. The setting range is from 1 to 10 and the power loss is low. However, a thermal memory cannot be simulated by the current transformer itself.
In the case of monitoring by way of a shunt, the shunt is integrated into the current path, so that voltage tapping to characterize the current flow can be carried out. Downstream analysis of the voltage across the shunt enables an imminent thermal overload to be detected. Using a shunt measurement method, detection of AC/DC currents is possible. The setting range is typically from 1 to 4. A disadvantage of the measuring method using a shunt is that a thermal memory cannot be simulated with voltage tapping at the shunt and the electrical isolation of the individual phases is possible only with great difficulty.